Variable displacement hydraulic motors, such as hydraulic axial piston motors, are widely used on earthworking and construction machines, e.g., loaders, excavators, dozers, and the like. For example, a typical application may be found on track loaders. On these vehicles, a first variable displacement hydraulic motor may be used to drive a right-side track and a second variable displacement hydraulic motor may be used to drive a left-side track. To keep the vehicle moving in a straight line, the output speed of each hydraulic motor must be precisely controlled so that the right and left tracks move at substantially the same speed.
In many conventional applications, linear control systems, such as hydro-mechanical control systems, are used to control the output speed of variable displacement hydraulic motors. However, variable displacement hydraulic motors are inherently non-linear systems, and linear control systems tend to have limited stable operating ranges. In order to insure the stability of such a control system, small feedback gains must be used. Moreover, stability of the control system may become a critical issue when a wide range of operation is desired, for example when a desired motor output speed changes significantly and periodically. In addition, due to limited design flexibility, hydro-mechanical control systems tend to suffer from slow system response times, large overshoot, and high manufacturing costs. Further, adjustment of such control systems is often time consuming and costly.
Other electro-hydraulic (E/H) control methods exist which may be used to control systems having essentially nonlinear behavior. For example, one of the most common methods of control is to first linearize a nonlinear system and then control the resultant linearized system. A common example of such a method involves a Taylor Series linearization, which linearizes a small portion of the system about an operating point, the small portion being essentially linear in nature. One drawback of such a method is that predictable performance is assured only if the system stays close to the particular point about which the system linearized.
The present invention is directed to overcoming one or more of the problems set forth above.